In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film.
A resist material in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength (increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in mass production. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter (energy higher) than these excimer lasers, such as electron beam, extreme ultraviolet radiation (EUV), and X ray.
Resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources.
As a resist material that satisfies these conditions, a chemically amplified composition is used, which includes a base material component that exhibits a changed solubility in a developing solution under the action of acid and an acid-generator component that generates acid upon exposure.
For example, in the case where the developing solution is an alkali developing solution (alkali developing process), a chemically amplified positive resist which contains, as a base component (base resin), a resin which exhibits increased solubility in an alkali developing solution under action of acid, and an acid generator is typically used. If the resist film formed using the resist composition is selectively exposed during formation of a resist pattern, then within the exposed portions, acid is generated from the acid-generator component, and the action of this acid causes an increase in the solubility of the resin component in an alkali developing solution, making the exposed portions soluble in the alkali developing solution. In this manner, the unexposed portions remain to form a positive resist pattern. The base resin used exhibits increased polarity by the action of acid, thereby exhibiting increased solubility in an alkali developing solution, whereas the solubility in an organic solvent is decreased. Therefore, when such a base resin is applied to a process using a developing solution containing an organic solvent (organic developing solution) (hereafter, this process is referred to as “solvent developing process” or “negative tone-developing process”) instead of an alkali developing process, the solubility of the exposed portions in an organic developing solution is decreased. As a result, in the solvent developing process, the unexposed portions of the resist film are dissolved and removed by the organic developing solution, and a negative resist pattern in which the exposed portions are remaining is formed.
However, in the case of positive tone development process in which a resist composition that forms a positive resist pattern by an alkali development process, when a trench pattern (isolated space pattern) or a hole pattern is formed, it becomes necessary to form a resist pattern using an incident light weaker than that used in the case of a line pattern or a dot pattern, such that the contrast of the intensity of the incident light between exposed portions and unexposed portions becomes unsatisfactory. Therefore, pattern formation performance such as resolution tends to be restricted, and it becomes difficult to form a resist pattern with a high resolution.
In contrast, a negative tone development process using a negative type, chemically amplified resist composition (i.e., a chemically amplified resist composition which exhibits decreased alkali solubility in an alkali developing solution upon exposure) in combination with an organic developing solution is advantageous over the positive tone development process in the formation of a trench pattern or a hole pattern. For this reason, in recent years, negative tone development process has been considered important for formation of fine patterns, and base resins and the like suitable for negative tone development process have been proposed.
For example, Patent Document 1 discloses a negative-tone development resist composition containing a resin having a structural unit derived from (meth)acrylate ester as a main chain, and a method of forming a pattern.
As miniaturization of resist pattern is required, there has been proposed methods in which a resist pattern is subjected to a thermal treatment to fluidize the pattern and downsize the pattern (see Patent Documents 2 to 4). For example, Patent Document 2 discloses a method in which a resist pattern is formed on a substrate, followed by a thermal treatment to change the cross-sectional shape of the resist pattern from a rectangular shape to a semicircular shape to increase the bottom length, thereby forming a minute pattern. Patent Document 3 discloses a method in which a resist pattern is formed, followed by heating the pattern around its softening temperature, so as to fluidize the resist to change the pattern size, thereby forming a minute pattern. Patent Document 4 discloses a method in which a positive resist composition containing a resin having a specific structure is used to form a resist pattern, followed by a thermal flow treatment, thereby forming a minute resist pattern.